CZT(S,Se) is an earth abundant, light absorbing material presently being used in photovoltaic devices. The highest efficiency photovoltaic devices have been achieved via growth of the absorber by solution deposition with the elemental constituents being dissolved in hydrazine. See, for example, U.S. Patent Application Publication Number 2013/0037090 filed by Bag et al., entitled “Capping Layers for Improved Crystallization.” CZT(S,Se)-based photovoltaic devices have been produced with efficiencies above 12% but these devices unfortunately suffer from point defects (crystallographic defects) which limit increases in open circuit voltage (Voc).
One approach to increasing Voc involves the use of high work function back contact materials to replace the standard molybdenum (Mo) metal contact. See, for example, U.S. Patent Application Publication Number 2013/0269764 filed by Barkhouse et al., entitled “Back Contact Work Function Modification for Increasing CZTSSe Thin Film Photovoltaic Efficiency” (hereinafter “U.S. Patent Application Publication Number 2013/0269764”). However, while the standard device thickness is generally about 2 micrometers (μm), contact engineering to achieve increases in Voc works only if the thickness of the CZTSSe absorber is reduced to below 1 μm. This critical dimension is determined by the sum of the back and front depletion widths and minority carrier diffusion length. See, for example, U.S. Patent Application Publication Number 2013/0269764. Above this value the device will not show improvement in Voc.
While sub-micron films can be fabricated with solution phase techniques, the resulting films suffer from pinholes. Pinholes are undesirable since they can lead to electrical shorting of the device when conductive contacts are added. Additionally, electron blocking layers or hole transport layers (often oxides) have been adopted in thin film solar cells for improved rectifying behavior and Voc. Oxides, however, get reduced by the hydrazine used in solution phase deposition, which is undesirable.
Therefore, techniques for forming high efficiency, defect-free CZTS(S,Se) absorber photovoltaic devices would be desirable.